California Avocado Society 1997 Yearbook 81: 117-148 ALTERNATE BEARING IN THE AVOCADO (PERSEA AMERICANA MILL.) Sergio Paz-Vega Master in Plant Sciences, University of California at Riverside. This paper is a slightly modified version of the author's approved thesis. INTRODUCTION An alternate or biennial bearing tree is one that does not bear a regular crop year after year. Instead, heavy yields are followed by extremely low ones and vice versa. Early literature emphasizes alternate or biennial bearing of deciduous fruit trees. However, this phenomenon is common in widely different families of trees including citrus, apples, mangoes, pistachios, pecans, and avocados52. During a single alternating cycle, the on-year is the year in which trees produce an intense bloom in the spring and a heavy crop load is set and carried through maturity, eventually resulting in a high yield at the time of harvest (on-harvest). An off-year is the year in which the trees produce a light bloom, resulting in a light crop load and a low yield at the time of harvest (off-harvest). Alternate or biennial bearing is not always an every other year cycle. An "extremely" on-year can be followed by one or more off-years and vice versa. Alternate bearing in avocado is a phenomenon that growers and the industry would like to eliminate because of its economic impact. In on-years, the large number of fruit produced per tree results in smaller individual fruit that are less valuable. In off-years, the fruit are of better size and quality, but the very low yields result in low net dollar return to the grower. Through the law of supply and demand there is a price fluctuation between on- and off-years. In addition, during the years of off-harvest it is frequently necessary to import fruit from other countries. This results in loss of market share. The initial trigger for the start of an alternating cycle can be an external factor such as frost, lack of pollination, or drought which greatly reduces one year's crop34. Inadequate thinning of the crop in years in which the high temperatures associated with June fruit drop do not occur can also trigger alternate bearing. Once this cycle has started, it usually is amplified annually and becomes self perpetuating68. Delayed harvest referred to as "on-tree storage" or "late hanging of fruit" has been proposed as an additional cause of alternate bearing in the avocado. The difference in yield between the on-year and the off-year can vary. Reductions of up to almost 100% in yield of the off-harvest relative to the on-harvest have been reported73. Alternate bearing in avocado can occur at different levels. In sub-tropical areas where climatic conditions are more moderated, it usually occurs at the orchard or tree level. However, because the biennial cycle usually starts in response to external factors, in some areas with more extreme climatic conditions alternate bearing occurs in most of the orchards; and it can affect the whole industry of a particular area. In California, as early as 1947 it was observed that the total production of the state exhibited alternating cycles in which, for a specific year, most of the orchards were in the on-year, followed by a year in which most of them were in the off-year29. The physiological and biochemical bases for alternate bearing in avocado are not well defined. Research thus far has emphasized two hypotheses to explain the perpetuation of alternate bearing, with little integration between the two. The first hypothesis suggests that flowering, fruit set, and fruit growth utilize large amounts of carbohydrate. Thus, a carbohydrate deficit would limit these processes and eventually result in alternate bearing. It is hypothesized that the heavy crop load in on-years would cause a reduction in the accumulation of carbohydrate during the autumn and winter, resulting in a light bloom the next spring characteristic of off-years. During an off-year, trees would accumulate higher carbohydrate levels during autumn and winter, resulting in an intense bloom the following on-year13,25. The second hypothesis was proposed by researchers who favor the role of plant growth regulators (PGRs) produced by the seed of the developing fruit, which during the on- year would inhibit flower initiation and cause the light bloom characteristic of the subsequent off-year. The total number of studies on PGRs in avocado is very small, likely due to the difficulties inherent in such studies. Thus, evidence for or against this hypothesis is limited for avocado. In this paper, the experimental evidence related to alternate bearing in the avocado that is reported in the literature is critically reviewed to determine whether there is sufficient evidence to support one or both of the hypotheses as an explanation for the perpetuation of the alternating cycle of yield. As a result of this critical review, an alternative hypothesis on alternate bearing is emphasized, which takes into consideration the effect of the crop load on the amount of current year summer and fall vegetative shoot growth as the key factor impacting the number of potential flowering points the following spring. In conclusion, a series of tree and orchard management practices are proposed that could be used by growers to eliminate alternate bearing and actually maximize tree yields on an annual basis. HYPOTHESIS 1: CARBOHYDRATE AVAILABILITY PERPETUATES ALTERNATE BEARING IN AVOCADO. Starch is the most common and important form of carbohydrate in most higher plants. It can accumulate to concentrations averaging 20% of the dry mass of some tissues. It is the most useful indicator of seasonal fluctuations in carbohydrate in crops and is closely related to aspects of tree performance of interest to horticulturists81. Starch concentrations in vegetative tissues are greater during the off-year. This has been repeatedly confirmed in apples28, pecans22, and citrus33. One of the best known studies favoring the hypothesis that carbohydrate availability is the basis for the perpetuation of alternate bearing was done by Scholefield et al.59. Their study identified the seasonal changes in starch concentrations in avocado branches and established that high yields were associated with a high accumulation of starch during the previous winter, yielding high starch levels in the spring at the onset of flowering. Maximum levels of starch occurred in late winter through early spring just before flowering, and declined during flowering, shoot growth, and fruit development to a minimum in late summer and autumn. Starch accumulation began at the time of cessation of vegetative growth and continued over winter and early spring. The heavy crop load in on-years resulted in low concentrations of starch the following winter and spring and a late light bloom, accompanied by much new vegetative growth. Thus, under the conditions where the experiment took place, alternate bearing appeared to be closely related to carbohydrate availability in the tree-contrary to the hypothesis that high carbohydrate concentration is required for floral initiation. The authors suggested that low levels of carbohydrate might have caused the cessation of vegetative growth prerequisite to floral initiation14. Interestingly, floral initiation was observed to occur at the time of minimum carbohydrate content of the major branches. Whiley and Wolstenholme71 found similar seasonal variations in starch levels; i.e., the greater accumulation occurred during autumn and winter reaching the highest concentration just prior to flowering. They also observed a reduction in starch levels during the period of flowering, fruit set, and fruit growth. The minimum starch concentration occurred during the summer. They included these changes in starch concentration in their phenological model for the 'Fuerte' avocado. The results of Scholefield et al.59, although better known, were similar to those of a much earlier study by Cameron and Borst15. They showed that a tree carrying a heavy crop accumulated less starch during the winter and had lower starch levels during summer and autumn than nonbearing trees. They suggested that starch storage during the autumn and early winter was reduced by the demands of the developing fruit. Evidence that crop load reduced carbohydrate availability was provided by the observation that trees produced practically no new vegetative growth during the summer that they were carrying a heavy crop. In a study done by Van der Walt et al.68, a close relation between carbohydrate levels and amount of fruit carried by the tree was found. Their data demonstrated the change in carbohydrate status of trees during the transition between an on- and off-year. They observed that the heavy fruit load depleted the carbohydrate reserves of on-trees in November and January, but that these same trees accumulated starch in May as a result of the low demand for carbohydrate by the light bloom and crop of the subsequent off-year. Conversely, off-trees accumulated starch in the winter and showed a sharp decline in the reserve content of carbohydrate during the spring when these trees produced a heavy bloom and set an on-crop. An additional argument that is used to support the hypothesis that carbohydrate availability perpetuates alternate bearing in avocado is the fact that avocado fruit contain high levels of oil (up to 40% of dry weight in some cultivars), which is obviously energy expensive and requires larger amounts of carbohydrate from the tree compared to sugar storing fruit like apples, oranges, or bananas. The high oil content of the avocado fruit may explain why carbohydrate reserves are severely depleted in an on- year during the period of fruit growth and oil accumulation. Using energy budgets, Wolstenholme78 calculated the potential yield of 'Fuerte' avocados to be only 32.5 tonnes/ha (at 17% oil content) compared to 100 tonnes/ha for apples and 88.5 tonnes/ha for 'Valencia' oranges.